Aneeqa Maria, from Christian advocacy group The Voice Society, has said Aleem Masih, 28, and his wife Nadia, 23, were ambushed by the latter's family members on the way to a doctor's appointment in the Youhanabad area of Lahore in July.
The couple had married last year, with Nadia (pictured above) leaving Islam and becoming a Christian in the process - something which angered her family.
The extremists then took the couple to a farm, beat them, then shot Aleem three times and Nadia once. Aleem died from his injuries, while Nadia is in a critical condition in hospital.
Nadia's family members then returned to their village where they publicly announced their crimes, before one of them - Azhar - handed himself into police.
Aleem and Nadia Masih had previously moved nearly 40 miles away from Lahore in order to keep themselves safe, and had also asked Lahore High Court for police protection because of threats from her family.
According to Aneeqa Maria, police have not yet detained any of the suspects because they have all been granted bail.
She told Morning Star News: "The attackers returned to their village and publicly proclaimed that they had avenged their humiliation and restored the pride of the Muslims by killing the couple in cold blood."
She added that a "... mob, some of them armed with weapons, was shouting furious anti-Christian slogans," outside the hospital where Nadia is recovering.
"They were also praising Azhar for restoring the pride of the Muslim Ummah [community] and saying that he had earned his place in paradise for killing an 'infidel'."
"Murder in the name of religion and honor must not be allowed, and the perpetrators should be held accountable for this brutal act."